The presently disclosed invention pertains to seismic surveying and, more particularly, to a technique for low-frequency, seismic acquisition.
Seismic surveying is the practice of probing subterranean formations in the Earth using sound waves. This includes imparting acoustic, or sound, waves into a natural environment so that they enter the earth and travel through the subterranean geological formations of interest. During their travels through the formations, certain features of the formations will return waves back to the surface where they are recorded.
The recorded returning wave energy is then studied to ascertain information about those formations. The seismic data derived from the recorded waves are processed to, for example, image the subterranean formations of interest. Frequently, the images are used to model the subterranean formations. For example, models known as “subsurface attribute models” are frequently developed to study the subterranean formations. The images, and models derived from them, can help identify subsurface resources. Most notably, these resources may include fluid hydrocarbons such as petroleum and natural gas. The techniques may be applied to the location of other kinds of resources as well.
One type of seismic survey is the “marine” seismic survey. The term “marine” only indicates that the survey occurs in or on the water. It does not necessarily imply that the survey is occurring in a saltwater environment. While a marine seismic survey may occur in a saltwater environment such as the ocean, it may also occur in brackish waters such as are found in bays, estuaries, and tidal swamps. They may even be conducted in wholly fresh waters such as are found in lakes, marshes, and bogs.
Another relatively recent development in seismic acquisition is “low-frequency” acquisition. Seismic surveying historically has used frequencies in the range of 6-80 Hz for seismic signals because of their suitability in light of technical challenges inherent in seismic surveying. The term “low frequencies” is understood within this historical context as frequencies below which getting sufficient signal to noise with conventional sources rapidly becomes more difficult as the frequency decreases, i.e. below about 6-8 Hz.
The use of low frequencies for imaging with marine seismic data has proven challenging for frequencies below about 6 Hz, particularly for frequencies below about 4 Hz. The challenge is twofold: at lower frequencies the naturally occurring seismic background noise of the Earth gets progressively stronger, and conventional broadband sources such as airguns get progressively weaker. As a result, the signal-to-noise of deepwater marine seismic data can decline at over 30 dB per octave for frequencies below 4 Hz.
Thus, while there may be many suitable techniques for seismic imaging in general and for generating subsurface attribute models in particular, the need for increased effective signal-to-noise at low frequencies in the acquisition of seismic data continues to drive innovation in the art. In particular, among other things, there is a need for acquisition and processing techniques that enhance acquisition and use of low-frequency seismic data at lower frequencies. The art is therefore receptive to improvements or at least alternative means, methods and configurations that might further the efforts at improvement.